Antistatic polyester film

ABSTRACT

An antistatic polyester film having formed on at least one side of a polyester film a coating layer which consists essentially of (a) 5 to 40% by weight of a phosphoric acid ester salt-based antistatic agent, (b) 1 to 20% by weight of a surfactant having an HLB value of 12 or less, (c) 40 to 90% by weight of at least one binder selected from acrylic copolymers and polyester copolymers, and (d) 3 to 25% by weight of fine particles having an average particle diameter of 10 to 500 nm and which has an extremely uniform surface. This film is excellent in anti-blocking properties at a high humidity and antistatic properties at a low humidity.

FIELD OF THE INVENTION

This invention relates to an antistatic polyester film and, morespecifically, to an antistatic polyester film having excellent coatingappearance, transparency, adhesion, anti-blocking properties at a highhumidity and antistatic properties at a low humidity.

PRIOR ART

Since a biaxially oriented polyester film has dominant featuresexcellent transparency, dimensional stability, mechanical properties,electrical properties and chemical resistance, it has been and is usedfor a wide range of applications.

However, an ordinary polyester film is liable to cause generation andaccumulation of static electricity because a polyester itself has highelectrical insulating properties, thereby causing various troubles dueto static electricity. For instance, a polyester film has been so farwidely used as an OHP (Over Head Projector) film, but a conventionalpolyester film for use as an OHP film is insufficient in antistaticproperties and hence, may cause such trouble that two or more films arecarried while they are overlapped with each other by staticelectrostatic charge when they are used in a copier.

There have been, heretofore, known methods for improving antistaticproperties such as one in which anionic compounds such as an organicsulfonic acid salt, a metal powder, carbon powder or the like is kneadedinto a substrate film and one in which a metal compound is vapordeposited on a substrate film. However, these methods involve suchproblems as a deterioration in transparency and an increase inprocessing costs.

Further, there is known another known method for improving antistaticproperties to form a coating film of a synthetic resin (binder) whichcontains an antistatic agent, on the surface of a substrate film inadvance. In this case, an anionic compound or a cationic compound havinggood application properties is used as the antistatic agent. However,these materials are inferior in antistatic properties at a low humidity.Although this defect can be eliminated to some extent by using a largeamount of the antistatic agent, a new problem occurs that films readilystick to each other (to be referred to as "blocking" hereinafter) at ahigh humidity due to the hydrophilic nature of the antistatic agent.Thus, it has been difficult for a conventional antistatic agent toachieve anti-blocking properties and antistatic properties at a lowhumidity at the same time.

JP-A 6-255055 (the term "JP-A" as used herein means an "unexaminedpublished Japanese patent application") and JP-B 1-49114 (the term"JP-B" as used herein means an "Japanese Laid-open Patent Publication")teach that a phosphoric acid-based antistatic agent is excellent inantistatic properties even at a low humidity and exhibits excellentconductivity and antistatic properties even in an in-line coating methodin which it is applied as a coating fluid to an unstretched ormonoaxially stretched polyester film and the coated film is stretchedand is subjected to heat-treatment. However, in this method, theapplication properties of the phosphoric acid-based antistatic agent areinsufficient due to its poor wettability to a polyester film substrateand good coating appearance cannot be obtained without pretreatment suchas corona discharge treatment of the film substrate. Therefore, theapplication of the antistatic agent has been limited.

Some of the inventors of the present invention have proposed in JP-A7-156357 an antistatic film for photoengraving, which has formed on itsone side or both sides a coating layer of a composition essentiallycomprising (A) 30 to 75% by weight of an aqueous polyester having asecondary transition point of 40 to 85° C., (B) 20 to 45% by weight ofphosphoric acid ester salt-based antistatic agent and (C) 0 to 30% byweight of fine particles, and a production process thereof. Thisantistatic film for photoengraving has excellent antistatic propertiesand transparency and good humidity resistance. The above publicationteaches that a surfactant can be added to the coating layer to improvethe wettability of the film surface, and in its Examples,polyoxyethylene nonylphenylether was used as the surfactant. Anantistatic film obtained in the Example in which this surfactant (havingan HLB value of 14.2) was used was insufficient in the uniformity of acoating layer formed on the surface and the generation of fine coatingspots was often observed.

SUMMARY OF THE INVENTION

The inventors of the present invention have conducted intensive studiesto solve the above problems and have found that, when a specificsurfactant and fine particles are combined with a phosphoric acid estersalt-based antistatic agent, the resulting mixture can be applied wellas a coating fluid without a pretreatment such as corona dischargetreatment, an uniform coating film can be formed, and a antistaticpolyester film having excellent coating appearance, antistaticproperties, anti-blocking properties and adhesion and being extremelyuseful can be produced at low costs. The present finding has led to thepresent invention.

According to the present invention, there is provided an antistaticpolyester film having formed on one side or both sides thereof a coatinglayer having composition which essentially consists of:

(a) 5 to 40% by weight of a phosphoric acid ester salt-based antistaticagent;

(b) 1 to 20% by weight of a surfactant having an HLB value of 12 orless;

(c) 40 to 90% by weight of at least one binder selected from acryliccopolymers and polyester copolymers; and

(d) 3 to 25% by weight of fine particles having an average particlediameter of 10 to 500 nm, the surface of the coating layer having suchuniformity that the reduction rate of the glossiness of the coatedpolyester film surface having aluminum vapor deposited thereon is 5% orless, based on the glossiness of an uncoated polyester film surfacehaving aluminum vapor deposited thereon when it is taken as 100%.

The present invention is described hereinafter in detail.

DETAILED DESCRIPTION OF THE INVENTION

The polyester for forming the polyester film in the present invention isa linear saturated polyester synthesized from an aromatic dibasic acidor its ester-forming derivative and a diol or its ester-formingderivative. Illustrative examples of the polyester include polyethyleneterephthalate, polyethylene isophthalate, polybutylene terephthalate,poly(1,4-cyclohexylenedimethyleneterephthalate),polyethylene-2,6-naphthalene dicarboxylate and the like. Copolymersthereof and blends thereof which contain a small amount of other resinare also included. Among these, polyethylene terephthalate andpolyethylene-2,6-naphthalene dicarboxylate are particularly preferred.

These polyesters may contain, as required, an inorganic lubricant-suchas silicon oxide, aluminum oxide, magnesium oxide, calcium carbonate,kaolin, talc, titanium oxide or barium sulfate; an organic lubricantsuch as a crosslinked silicone resin, crosslinked polystyrene resin,crosslinked acrylic resin, urea resin or melamine resins; other resinsuch as polyethylene, polypropylene, ethylene-propylene copolymer orolefinic ionomer; and other additive such as an antioxidant, ultravioletabsorber or fluorescent brightener.

In the present invention, the polyester film is a biaxially orientedfilm, preferably a transparent biaxially oriented film. The thickness ofthe film is 5 to 500 μm, preferably 20 to 500 μm, the most preferably 50to 300 μm. When the thickness is less than 5 μm, the film is fragile andhence, not suitable. When the thickness is more than 500 μm, the filmforming property is liable to deteriorate.

In the present invention, the antistatic coating layer to be formed onone side or both sides of the polyester film has composition whichessentially consists of the following components (a) to (d):

(a) 5 to 40% by weight of a phosphoric acid ester salt-based antistaticagent;

(b) 1 to 20% by weight of a surfactant having an HLB value of 12 orless;

(c) 40 to 90% by weight of at least one binder selected from acryliccopolymers and polyester copolymers; and

(d) 3 to 25% by weight of fine particles having an average particlediameter of 10 to 500 nm.

The coating film having the above composition is formed on the surfaceof a polyester film substrate as an uniform film by preparing an aqueouscoating having this composition, applying the coating onto the surfaceof the polyester film substrate, drying and stretching the coating film.The coating film formed on the surface of the polyester film substrateis made extremely uniform by using the aqueous coating having the abovecomposition. When the degree of uniformity of the coating film isexpressed by its glossiness obtained when aluminum is vapor deposited onthe surface, a reduction in glossiness is 5% or less, preferably 4% orless.

That is, when aluminum is vapor deposited on a polyester film having nocoating film and a polyester film having a coating film formed thereonand these polyester films are compared in accordance with a measurementmethod to be described hereinafter, the difference of glossiness betweenthe two films is 5% or less. Therefore, it is understood that thepolyester film of the present invention has an extremely uniformsurface.

Conventionally known compounds can be used as the phosphoric acid estersalt-based antistatic agent (a) constituting the coating used in thepresent invention. Among the compounds, phosphoric acid ester salt-basedlow molecular weight compounds such as alkyl phosphate sodium salt,alkyl phosphate potassium salt, aryl phosphate sodium salt, arylphosphate potassium salt, dialkyl phosphate sodium salt, dialkylphosphate potassium salt, diaryl phosphate sodium salt, diaryl phosphatepotassium salt, polyalkylene oxide phosphate sodium salt, polyalkyleneoxide phosphate potassium salt, substituted phosphoric partial estersalt and the like are preferably used. They may be used alone or incombination of two or more.

These phosphoric acid ester salt-based antistatic agents (a) are used inthe coating layer in a proportion of 5 to 40% by weight, preferably 10to 30% by weight. If the proportion is more than 40% by weight,anti-blocking properties and adhesion will be insufficient and if theproportion is less than 5% by weight, an antistatic effect will beinsufficient.

In the coating layer of the present invention, the surfactant (b) has anHLB value of 12 or less. The term "HLB" stands for Hydrophile-LipophileBalance which indicates the balance between a lipophilic portion andhydrophilic portion contained in the molecule of a surfactantnumerically and which has been proposed by Mr. Griffin of Atlas Co. TheHLB value can be calculated from the following equation (I).

    HLB=20 (1-Mo/M)

wherein Mo is a molecular weight of a hydrophobic group and M is amolecular weight of the surfactant.

In the present invention, the phosphoric acid ester salt-basedantistatic agent (a) contained in the coating layer has such a defectthat it readily forms a spotted coating film on the surface of ahydrophobic polyester film due to its high hydrophilic nature. However,when the above antistatic agent is used in combination with the abovesurfactant having a low HLB value, an uniform coating film can be formedbecause this hydrophilic nature seems to be alleviated. When asurfactant having an HLB value of more than 12 is used, its hydrophilicnature becomes strong and a spotted coating film is liable to be formed.

Illustrative examples of the surfactant having an HLB value of 12 orless include a block copolymer of polyethylene oxide and polypropyleneoxide, polyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether,polyoxyethylene monoalkylate, polyoxyethylene aliphatic acid ester,sorbitan aliphatic acid ester, glycerin aliphatic acid ester and thelike. They may be combined with another surfactant having an HLB valueof more than 12 in an amount that does not generate coating spots. TheHLB value of the surfactant (b) is preferably in the range of 12 to 5,particularly preferably 11.5 to 8. The surfactant (b) is used in thecoating layer in a proportion of 1 to 20% by weight, preferably 3 to 15%by weight. If the proportion is more than 20% by weight, anti-blockingproperties will be insufficient and if the proportion is less than 1% byweight, the wettability of the coating will be insufficient, therebymaking it difficult to form an uniform film.

An acrylic copolymer or a polyester copolymer is used as the binder (c)in the coating layer of the present invention. A mixture of thesecopolymers may be used. The coating layer of the present invention isformed by applying an aqueous coating prepared by dissolving ordispersing coating constituents (a) to (d) in an aqueous medium(typically water), to the surface of a film. Therefore, the copolymer ofthe binder component (c) preferably is soluble or readily dispersible inan aqueous medium.

A monomer component constituting the acrylic copolymer used as thebinder (c) is exemplified by acrylic acid, methyl acrylate, ethylacrylate, butyl acrylate, soda acrylate, ammonium acrylate,2-hydroxyethyl acrylate, methacrylic acid, methyl methacrylate, ethylmethacrylate, butyl methacrylate, soda methacrylate, ammoniummethacrylate, 2-hydroxyethyl methacrylate, glycidyl methacrylate,acrylamide, methacrylamide, N-methylolacrylamide,N-methylolmethacrylamide and the like. These monomers may be used incombination with other unsaturated monomers such as styrene, vinylacetate, acrylonitrile, methacrylonitrile, vinyl chloride, vinylidenechloride and divinyl benzene. A block polymer or graft polymer obtainedby modifying an acrylic copolymer with a polyester, polyurethane,silicone, epoxy or phenolic resin or the like may be used as the acryliccopolymer.

Among the above monomers constituting the acrylic copolymer, ethylacrylate, methacrylic acid, methyl methacrylate, 2-hydroxyethylmethacrylate and N-methylolacrylamide are preferred.

The acrylic copolymer preferably has a secondary transition temperature(Tg) of 25 to 120° C., particularly preferably 40 to 85° C. This acryliccopolymer preferably has a molecular weight of about 100,000 to about1,000,000, particularly preferably about 200,000 to about 800,000. Whenthe secondary transition temperature (Tg) of the acrylic copolymer isless than 25° C., the anti-blocking properties of a coated filmdeteriorate and when Tg is more than 120° C., it is difficult to form anuniform coating film. When the molecular weight of the acrylic copolymeris smaller than about 100,000, it is difficult to form an uniformcoating film and the film is fragile even if it is formed. When themolecular weight is more than about 1,000,000, it is not easy to coat afilm.

An acid component forming the polyester copolymer as the bindercomponent (c) is exemplified by terephthalic acid,2,6-naphthalenedicarboxylic acid, isophthalic acid, phthalic acid,phthalic anhydride, 1,4-cyclohexanedicarboxylic acid, adipic acid,sebacic acid, phenylindanedicarboxylic acid, dimer acid and the like.These components may be used in combination of two or more. Among theseacid components, terephthalic acid, 2,6-naphthalenedicarboxylic acid andisophthalic acid are preferred. Further, these components may be used incombination with a small amount of an unsaturated polybasic acid such asmaleic acid, fumaric acid and itaconic acid; and hydroxycarboxylic acidsuch as p-hydroxybenzoic acid and p-(β-hydroxyethoxy)benzoic acid. Theproportion of the unsaturated polybasic acid component or thehydroxycarboxylic acid component is 10 mol % at the most, preferably 5mol % or less.

A polyol component forming the polyester copolymer is exemplified byethylene glycol, 1,4-butane diol, neopentyl glycol, diethylene glycol,dipropylene glycol, 1,6-hexane diol, 1,4-cyclohexane dimethanol,xylylene glycol, dimethylolpropionic acid, glycerin, trimethylolpropane, poly(ethyleneoxide)glycol, poly(tetramethyleneoxide)glycol, andadduct of bisphenol A and ethylene oxide and adduct of bisphenol A andpropylene oxide, each represented by the following formulas: ##STR1##wherein n+m=2 to 10 and ph=phenylene group.

They may be used in combination of two or more.

Among these polyol components, ethylene glycol, adduct of bisphenol Aand ethylene oxide, adduct of bisphenol A and propylene oxide and1,4-butane diol are preferred, and ethylene glycol and adduct ofbisphenol A and ethylene oxide and adduct of bisphenol A and propyleneoxide are more preferred.

A trace amount of a compound having a sulfonic acid base or a compoundhaving a carboxylic acid base is preferably copolymerized to facilitatewater solubilization of the polyester copolymer.

Illustrative examples of the compound having a sulfonic acid base forfacilitating water solubilization of the polyester copolymer includecompounds containing alkali metal salts of sulfonic acid and amine saltsof sulfonic acid such as 5-sodium sulfoisophthalic acid, 5-ammoniumsulfoisophthalic acid, 4-sodium sulfoisophthalic acid, 4-methylammoniumsulfoisophthalic acid, 2-sodium sulfoisophthalic acid, 5-potassiumsulfoisophthalic acid, 4-potassium sulfoisophthalic acid, 2-potassiumsulfoisophthalic acid and sodium sulfosuccinic acid.

Illustrative examples of the compound having a carboxylic acid baseinclude trimellitic anhydride, trimellitic acid, piromellitic anhydride,pyromellitic acid, trimesic acid, cyclobutane tetracarboxylic acid,dimethylolpropionic acid and monoalkali metal salts thereof.

It is advantageous that these sulfonic acid base containing compoundsand carboxylic acid base containing compounds are copolymerized in thepolyester copolymer in a proportion of 2 to 30 mol %, preferably 5 to 20mol %.

A modified polyester copolymer such as a block copolymer or graftcopolymer prepared by modifying a polyester copolymer with an acrylic,polyurethane, silicone, epoxy or phenolic resin or the like may be usedas the polyester copolymer.

The polyester copolymer or the acrylic copolymer of the binder (c) inthe present invention is easily adhered to a toner for OHP, for example.From a viewpoint of this adhesion function, the polyester copolymer ismore preferred than the acrylic copolymer.

The polyester copolymer preferably has a secondary transitiontemperature (Tg) of 25 to 120° C., particularly preferably 40 to 85° C.The polyester copolymer preferably has a molecular weight of about 3,000to about 80,000, particularly preferably about 10,000 to about 50,000.

The binder (c) is used in the coating layer in a proportion of 40 to 90%by weight, preferably 50 to 80% by weight. When the proportion of thebinder (c) is less than 40% by weight, the adhesion of the coating layeris insufficient, while when the proportion is more than 90% by weight,an antistatic effect is unsatisfactory disadvantageously.

The fine particles (d) in the coating layer of the present inventionhave an average particle diameter of 10 to 500 nm and may be inorganicor organic fine particles. Illustrative examples of the inorganic fineparticles include fine particles of calcium carbonate, calcium oxide,aluminum oxide, kaolin, silicon oxide and zinc oxide, and those of theorganic fine particles include fine particles of a crosslinked acrylicresin, crosslinked polystyrene resin, melamine resin, fluororesin,crosslinked silicone resin and polyolefin. The particle diameter of thefine particles is 10 to 500 nm, preferably 20 to 300 nm. When theparticle diameter is less than 10 nm, anti-blocking properties is liableto deteriorate, while the particle diameter is more than 500 nm,transparency is liable to lower.

The fine particles (d) are used in the coating layer in a proportion of3 to 25% by weight, preferably 5 to 20% by weight. When the proportionis more than 25% by weight, the transparency of the resulting filmlowers, while when the proportion is less than 3% by weight, theanti-blocking properties of the resulting film deterioratedisadvantageously.

In the present invention, the antistatic coating film on the surface ofthe polyester film is formed by applying an aqueous coating havingcomposition comprising (a) to (d) above to a film substrate. Althoughwater is substantially used in the aqueous coating as a medium, awater-soluble organic solvent may be contained in the coating, inaddition to water. Illustrative examples of the water-soluble organicsolvent include methyl ethyl ketone, acetone, ethyl acetate,tetrahydrofuran, dioxane, cyclohexanone, n-hexane, toluene, xylene,methanol, ethanol, n-propanol, isopropanol and the like. They may beused alone or in combination of two or more.

The coating may contain other additive such as an ultraviolet absorber,pigment, anti-foaming agent, coating improving agent, lubricant,anti-blocking agent, melamine-based crosslinking agent, urea-basedcrosslinking agent, guanamine-based crosslinking agent, epoxy-basedcrosslinking agent, aziridine-based crosslinking agent or blockisocyanate-based crosslinking agent, coupling agent or the like.

The solid content concentration of the aqueous coating in the presentinvention is generally 30% by weight or less, preferably 20% by weightor less. This solid content concentration is preferably 0.4% by weightor more.

The term "solid content" as used herein denotes components other thanthe solvent and is substantially the total amount of the components (a)to (d).

In the present invention, the coating comprising the components (a) to(d) is applied to at least one side of the polyester film substratewhich is a polyester film whose crystalline orientation has not beencompleted yet.

The polyester film substrate whose crystalline orientation has not beencompleted yet is exemplified by an unoriented film prepared bymelt-extruding a polyester and forming the resulting polyester into aform of film, a monoaxially oriented film prepared by stretching anunoriented film in a longitudinal or transverse direction, and abiaxially oriented film prepared by stretching in both longitudinal andtransverse directions at low draw ratios (a biaxially oriented filmwhich is not re-oriented in a longitudinal or transverse direction tocomplete its crystalline orientation). Among these, a monoaxiallyoriented film which is stretched in a longitudinal direction ispreferred.

Any known application method can be employed as the method for applyingthe coating to the polyester film substrate. For example, a rollcoating, gravure coating, microgravure coating, reverse coating, rollbrushing, spray coating, air knife coating, impregnation and curtaincoating may be employed alone or in combination. The aqueous coating maycontain a trace amount of the organic solvent to promote the stabilityof the coating and the application properties of the coating.

The amount of coating is preferably 0.5 to 50 g, more preferably 5 to 30g per m² of the traveling film. The thickness of the final dried coatingfilm needs to be 0.01 to 1 μm, preferably 0.02 to 0.6 μm. When thethickness of the coating film is less than 0.01 μm, sufficientantistatic properties cannot be obtained. When the thickness is morethan 1 μm, anti-blocking properties deteriorates disadvantageously.

Coating may be carried out on one side only or both sides of the filmaccording to the application of the film. After coating, drying iscarried out to obtain an uniform coating film.

In the present invention, after the coating is applied to the polyesterfilm substrate, the resulting film is dried and oriented. This drying ispreferably carried out at 90 to 130° C. for 2 to 20 seconds. This dryingcan also function as a preheat treatment for orientation or a heattreatment at the time of orientation.

The polyester film is drawn at a temperature of 70 to 140° C. to 2.5 to7 times in a longitudinal direction and to 2.5 to 7 times in atransverse direction with an area magnification of 8 times or more,preferably 9 to 28 times. When the film is re-oriented, it is preferablydrawn to 1.05 to 3 times (with the same area magnification). A thermalsetting treatment after orientation is preferably carried out at atemperature higher than the final orientation temperature and lower thana melting point for 1 to 30 seconds. For example, a polyethyleneterephthalate film is preferably thermally set at 170 to 240° C. for 2to 30 seconds.

The thus obtained antistatic polyester film has a light transmittance of90% or more and a surface resistivity (at a temperature of 23° C. and ahumidity of 35%) of 1×10¹² Ω·square⁻¹ or less, preferably 1×10¹⁰ to1×10¹¹ Ω·square⁻¹, is excellent in coating appearance, transparency,adhesion, humidity resistance at a high humidity and antistaticproperties at a low humidity, has an extremely uniform coating film, andis advantageously usable as a film for magnetic tape, display, OHP,printing, photoengraving, packaging and the like. Particularly, it isextremely useful as a film for OHP or photoengraving.

EXAMPLES

The following examples are given to further illustrate the presentinvention. Characteristic properties in the examples are obtained by thefollowing methods.

1. coating appearance of coated film

The coating appearance of a coated film is evaluated visually. A coatingsurface which is uniform without defect is evaluated as ∘ and a coatingsurface having spots or marble-like defects is evaluated as X.

2. light transmittance (transparency)

Measured in accordance with ASTM D1003 using the Model HR-100 haze meterof Murakami Color Research Laboratory Co. Ltd.

3. friction coefficient (slipperiness)

A static coefficient of friction (μs) is measured using the slipperinessmeasuring instrument of Toyo Tester Industry Co. Ltd., in accordancewith ASTM D1894-63 by putting a front surface of a film and a rearsurface of another film together and applying a load of 1 kg.

4 anti-blocking properties (humidity resistance)

Peel strength (g/50 mm) is measured at a portion which is applied a loadby a tensile tester after the front and rear surfaces of a 50 mm-widesample film are put together and treated for 17 hours under a load of 50kg/cm² at 60° C. and 80% RH. Anti-blocking properties is evaluated asfollows from the value of peel strength.

good anti-blocking properties: peel strength≦10

fair anti-blocking properties: 10<peel strength≦30

poor anti-blocking properties: 30<peel strength

5 toner adhesion (adhesion)

Data is printed on a coated film using a copier (Model Vivace 500) ofFuji Xerox Co. Ltd. to observe and evaluate the adhesion of toner.

6. surface resistivity (antistatic properties)

The surface resistivity (Ω·square⁻¹) of a sample film is measured usinga resistivity measuring instrument of Takeda Riken Industry Co. Ltd. ata temperature of 23° C. at a humidity of 35% 1 minute after a voltage of500 V is applied. The surface resistivity is preferably 1×10¹¹Ω·square⁻¹ or less.

7. uniformity of coating film (reduction rate of glossiness)

Aluminum is vapor deposited on the surface of a coated film (thethickness of aluminum deposited is 700Å) and the glossiness of theresulting film surface is measured at 85° C. using the Multi-Gloss 268glossimeter of Minolta Co. Ltd. Separately, aluminum is vapor depositedon the surface of an uncoated film and the glossiness of the resultingfilm surfaces is measured under the same conditions. When the glossinessof the uncoated film surface is taken as 100%, a relative value ofglossiness of the coated film surface is obtained. A relative glossinessvalue of more than 95% is equivalent to a reduction rate of 5% or less.

To vapor deposit aluminum on the surface of an uncoated film, theuncoated film is oriented under the same conditions as those of a coatedfilm and then, aluminum is vapor deposited on the surface of theuncoated film under the same conditions as those of the coated film. Theglossiness of the uncoated film surface is measured and taken as 100%.As a simple method which is only applied to a film coated on one side,aluminum is vapor deposited on a surface opposite to the coated surfaceof the film, the glossiness of the uncoated surface is measured andtaken as 100% to obtain a relative value.

Example 1

A copolyester (Tg=77° C.) having an intrinsic viscosity of 0.51 whichwas prepared from terephthalic acid, isophthalic acid, 5-sodiumsulfoisophthalic acid, ethylene glycol and adduct of bisphenol A andethylene oxide represented by the following structural formula wasdispersed in an aqueous medium to prepare an aqueous dispersion having asolid content concentration of 5% as an aqueous solution A. ##STR2##

wherein n+m=4 and ph=1,4-phenylene group.

Separately, a polyester (intrinsic viscosity: 0.65) prepared fromterephthalic acid and ethylene glycol was melt-extruded over a rotarycooling drum which was maintained at 20° C. to produce an unorientedfilm. Then the unoriented film was drawn to 3.6 times in a machinedirection and an aqueous coating prepared by mixing 60% by weight of theabove aqueous solution A, 20% by weight of an aqueous solution(concentration: 5% by weight) of dipentyl phosphate sodium salt as anantistatic agent, 10% by weight of an aqueous solution (concentration:5% by weight) of polyoxyethylene nonylether (HLB: 11.3) as a surfactantand 10% by weight of an aqueous dispersion (concentration: 5% by weight)of crosslinked acrylic resin particles (particle diameter: 60 nm) wasapplied to both sides of the film in an amount of 5 g/m² (wet) by akiss-roll coating method. Subsequently, the film was drawn to 3.9 timesin a transverse direction to produce a 100 μm-thick coated biaxiallyoriented polyester film.

The coating appearance, light transmittance, friction coefficient,anti-blocking properties, toner adhesion and surface resistivity of thisfilm are shown in Table 1.

Example 2

An acrylic copolymer (number average molecular weight: 263,000) preparedfrom methyl methacrylate, ethyl acrylate, 2-hydroxyethyl methacrylateand N-methylolmethacrylamide was dispersed in an aqueous medium toprepare an aqueous dispersion having a solid content concentration of 5%as an aqueous solution B.

A coated biaxially oriented polyester film was obtained in the samemanner as in Example 1 except that the aqueous solution A used inExample 1 was changed to the above aqueous solution B.

The coating appearance, light transmittance, friction coefficient,anti-blocking properties, toner adhesion and surface resistivity of thisfilm are shown in Table 1.

Example 3

A coated biaxially oriented polyester film was obtained in the samemanner as in Example 1 except that the surfactant used in Example 1 waschanged to polyoxyethylene nonylphenyl ether (HLB: 10.7).

The coating appearance, light transmittance, friction coefficient,anti-blocking properties, toner adhesion and surface resistivity of thisfilm are shown in Table 1.

Example 4

A coated biaxially oriented polyester film was obtained in the samemanner as in Example 1 except that the aqueous coating used in Example 1was changed to an aqueous coating prepared by mixing 35% by weight ofthe aqueous solution A, 30% by weight of the aqueous solution B, 20% byweight of an aqueous solution (concentration of 5% by weight) of dioctylphosphate potassium salt as an antistatic agent, 10% by weight of anaqueous solution (concentration: 5% by weight) of polyoxyethylenenonylphenyl ether (HLB: 10.8) as a surfactant and 5% by weight of anaqueous dispersion (concentration: 5% by weight) of crosslinkedpolystyrene resin particles (particle diameter: 120 nm).

The coating appearance, light transmittance, friction coefficient,anti-blocking properties, toner adhesion and surface resistivity of thisfilm are shown in Table 1.

Comparative Example 1

A coated biaxially oriented polyester film was obtained in the samemanner as in Example 1 except that the surfactant used in Example 1 waschanged to polyoxyethylene nonylphenyl ether (HLB: 14.2).

The coating appearance, light transmittance, friction coefficient,anti-blocking properties, toner adhesion and surface resistivity of thisfilm are shown in Table 1.

Comparative Example 2

A coated biaxially oriented polyester film was obtained in the samemanner as in Example 1 except that the coating composition used inExample 1 was changed to a coating composition consisting of 30% byweight of the aqueous solution A, 50% by weight of an aqueous solution(concentration of 5% by weight) of dipentyl phosphate potassium salt asan antistatic agent, 10% by weight of an aqueous solution(concentration: 5% by weight) of polyoxyethylene nonylphenyl ether (HLB:10.7) as a surfactant and 10% by weight of an aqueous dispersion(concentration: 5% by weight) of crosslinked acrylic resin particles(particle diameter: 60 nm).

The coating appearance, light transmittance, friction coefficient,anti-blocking properties, toner adhesion and surface resistivity of thisfilm are shown in Table 1.

Comparative Example 3

A coated biaxially oriented polyester film was obtained in the samemanner as in Example 1 except that the coating composition used inExample 1 was changed to a coating composition consisting of 70% byweight of the aqueous solution A, 20% by weight of an aqueous solution(concentration of 5% by weight) of dipentyl phosphate potassium salt asan antistatic agent and 10% by weight of an aqueous solution(concentration: 5% by weight) of polyoxyethylene nonylphenyl ether (HLB:10.7) as a surfactant.

The coating appearance, light transmittance, friction coefficient,anti-blocking properties, toner adhesion and surface resistivity of thisfilm are shown in Table 1.

Comparative Example 4

A coated biaxially oriented polyester film was obtained in the samemanner as in Example 1 except that the antistatic agent used in Example1 was changed to dodecyl soda sulfonate.

The coating appearance, light transmittance, friction coefficient,anti-blocking properties, toner adhesion and surface resistivity of thisfilm are shown in Table 1.

Comparative Example 5

The light transmittance, friction coefficient, anti-blocking properties,toner adhesion and surface resistivity of a film obtained withoutapplication of a coating in Example 1 are shown in Table 1.

                                      TABLE 1    __________________________________________________________________________                                        anti-blocking                                                     surface resistivity                  reduction rate of                           light  friction                                        properties   (antistatic            Coating                  glossiness of coated                           transmittance                                  coefficient                                        (humidity    properties)            appearance                  surface (%)                           (transparency)                                  (slipperiness)                                        resistance)                                              toner adhesion                                                     (Ω ·                                                     square.sup.-1)    __________________________________________________________________________    Example No. 1            ◯                  3.1      93     0.42  good  good   5 × 10.sup.10    Example No. 2            ◯                  4.0      91     0.39  good  good   7 × 10.sup.10    Example No. 3            ◯                  3.4      93     0.43  good  good   5 × 10.sup.10    Example No. 4            ◯                  4.2      90     0.40  good  good   6 × 10.sup.10    Comp. Ex. No. 1            X     7.5      90     0.41  fair  good   5 × 10.sup.10    Comp. Ex. No. 2            ◯                  3.8      92     0.42  poor  poor   1 × 10.sup.10    Comp. Ex. No. 3            ◯                  3.0      94     0.60  poor  good   6 × 10.sup.10    Comp. Ex. No. 4            ◯                  3.4      92     0.42  fair  poor   >10.sup.13    Comp. Ex. No. 5            --    0.0      94     0.62  good  poor   >10.sup.13    __________________________________________________________________________

EFFECT OF THE INVENTION

Since an antistatic coating film having specific composition is formedon at least one side of a polyester film in the present invention, it ispossible to provide an antistatic polyester film, particularly for useas an OHP or photoengraving film, having excellent uniform coatingappearance, anti-blocking properties at a high humidity and antistaticproperties at a low humidity.

What is claimed is:
 1. An antistatic polyester film having formed on oneside or both sides thereof a coating layer having composition whichconsists essentially of:(a) 5 to 40% by weight of a phosphoric acidester salt antistatic agent; (b) 1 to 20% by weight of a surfactanthaving an HLB value of 12 or less; (c) 40 to 90% by weight of at leastone binder selected from acrylic copolymers and polyester copolymers;and (d) 3 to 25% by weight of fine particles having an average particlediameter of 10 to 500 nm, the surface of the coating layer having suchuniformity that the reduction rate of the glossiness of the coatedpolyester film surface having aluminum vapor deposited thereon is 5% orless, based on the glossiness of an uncoated polyester film surfacehaving aluminum vapor deposited thereon when it is taken as 100%.
 2. Anantistatic polyester film according to claim 1, wherein the amount ofthe phosphoric acid ester salt antistatic agent (a) in the coating layeris 10 to 30% by weight.
 3. An antistatic polyester film according toclaim 1, wherein the amount of the surfactant (b) in the coating layeris 3 to 15% by weight.
 4. An antistatic polyester film according toclaim 1, wherein the amount of the binder (c) in the coating layer is 50to 80% by weight.
 5. An antistatic polyester film according to claim 1,wherein the amount of the fine particles (d) in the coating layer is 5to 20% by weight.
 6. An antistatic polyester film according to claim 1,wherein the HLB value of the surfactant (b) is in the range of 5 to 12.7. An antistatic polyester film according to claim 1, wherein the binder(c) is a polyester copolymer.
 8. An antistatic polyester film accordingto claim 1, wherein the thickness of the coating layer is 0.01 to 1 μm.9. An antistatic polyester film according to claim 1 which has a lighttransmittance of 90% or more.
 10. A film for an overhead projector orphotoengraving which is formed from the antistatic polyester film ofclaim
 1. 11. A method for producing the antistatic polyester film ofclaim 1 comprising the steps of:applying an aqueous coating containing acomposition which consists essentially of:(a) 5 to 40% by weight of aphosphoric acid ester salt-based antistatic agent; (b) 1 to 20% byweight of a surfactant having an HLB value of 12 or less; (c) 40 to 90%by weight of at least one binder selected from acrylic copolymers andpolyester copolymers; and (d) 3 to 25% by weight of fine particleshaving an average particle diameter of 10 to 500 nm, onto one side orboth sides of a polyester film substrate; drying the resulting film; anddrawing the dried film.